Hitherto known as communication equipments are wired or wireless mobile equipments such as cellular phones and PHSs. A communication system has been proposed, in which any mobile equipment can perform two-way communication with the fixed telephone linked to it or with any other communication equipments, through a telephone network including telephone radio relay units installed by NTT DoCoMo. Inc. or the like. (See, for example, Jpn. Pat. Appln. Laid-Open Publication No. 2002-300074.)
In the case of the two-way communication described above, however, a loop is formed due to the electrical coupling, including sound waves that propagate in the space between the transmitting unit and receiving unit of each communication equipments. Consequently, oscillation or echoing may develop in some cases. The oscillation occurs when the loop gain of the electrical coupling is 1 or more. The echoing takes place when the loop gain of the electrical coupling is 1 or less. Therefore, the transmitting unit and the receiving unit cannot be integrated or combined. In practice, these units can hardly be positioned close to each other. This makes a great problem to the noise reduction, miniaturization, weight reduction, manufacturing-cost reduction, and the like of the communication equipments, no matter whether the equipment is wired or wireless.
In view of the foregoing, an object of the present invention is to provide an oscillation-echo preventing circuit and a microphone/speaker unit, which can solve the problem with the conventional system, can suppress not only oscillation but also echoing in two-way communication equipments and one-way communication equipments, thereby to ensure a stable communication.